It has been known that a conventional active type optical distance measuring sensor irradiates light from a light source for projection such as an LED (Light Emitting Diode), etc., to an object and outputs a signal responsive to the distance to the object by detecting reflection light from the object by means of an optical detecting element. For example, a PSD (Position Sensitive Detector) has been known as an optical distance measuring sensor for optical triangulation, which is capable of easily measuring the distance to an object. However, in recent years, in order to carry out more accurate distance measurement, it is expected that an optical TOF (Time-Of-Flight) type optical distance measuring sensor will be developed.
Further, an image sensor that is capable of simultaneously acquiring distance information and image information by the same chip has been demanded for installation in a vehicle or for automatic production systems in a factory. If an image sensor is installed at the front side of a vehicle, it is expected that the image sensor will be used for detection and recognition of a preceding vehicle and pedestrians. Also, an image sensor that acquires distance information consisting of single distance information or a plurality of distance information separately from image information is expected. It is preferable that the TOF method is used for such a distance measuring sensor.
In the TOF method, pulse light is emitted from a light source for projection to an object, and the pulse light reflected from the object is detected by an optical detecting element, wherein a difference in time between emission timing and detection timing of the pulse light is measured. Since the time difference (Δt) is the time required for the pulse light to fly over the distance (2×d), which is equivalent to two times of the distance d to the object, at the light speed (=c), d=(c×Δt)/2 is established. The time difference (Δt) can be said to be a phase difference between an emission pulse from the light source and a detection pulse. If the phase difference is detected, the distance d to the object can be obtained.
A light source that emits light in a near-infrared band has been frequently used as the light source for projection. Since the wavelength in the near-infrared band is similar to the visible light band than to the wavelength of the far-infrared band, it is possible to condense or image by using an optical system such as lenses. In addition, since the energy density of the near-infrared optical components included in the spectrum of sunlight is less than the visible light components, it becomes possible to detect the near-infrared optical components at a high S/N ratio by using a visible light cut filter in a state where the ratio, brought about by sunlight, of the near-infrared optical components detected by an optical detecting element is decreased.
Attention has been directed to an image sensor of a charge distribution system as an optical detecting element to carry out distance measurement by the TOF method. That is, with an image sensor of a charge distribution system, pulse-like charge that is generated in the image sensor in response to incidence of detection pulses is distributed to one potential well for an ON period of emission pulses and is distributed to another potential well for an OFF period. In this case, the ratio of the left and right distributed charges is proportionate to the phase difference between the detection pulse and the emission pulse, that is, the time required for the pulse light to fly over the distance, which is equivalent to two times the distance to an object, at light speed. Further, various types of charge distribution methods can be taken into consideration.
Also, Patent Document 1 referred to below discloses a distance measuring sensor that carriers out measurement of a distance by picking up signals based on delay time when repeated pulse light projected from a light source is reflected from an object to be measured.
Patent Document 1: Japanese Published Unexamined Patent Application No. 2005-235893